The cluster is dominated byhot blue luminous stars that have formed within the last 100 million years.Reflection nebulae around the brightest stars were once thought to be leftover material from their formation, but are now considered likely to be an unrelated dust cloud in theinterstellar medium through which the stars are currently passing.[11] This dust cloud is estimated to be moving at a speed of approximately 18 km/s relative to the stars in the cluster.[12]
Computer simulations have shown that the Pleiades were probably formed from a compact configuration that once resembled theOrion Nebula.[13] Astronomers estimate that the cluster will survive for approximately another 250 million years, after which the clustering will be lost due to gravitational interactions with the galactic neighborhood.[14]
Together with the open star cluster of theHyades, the Pleiades form theGolden Gate of the Ecliptic. The Pleiades have been said to "resemble a tiny dipper," and should not be confused with the "Little Dipper," or Ursa Minor.[15]
The name, Pleiades, comes fromAncient Greek:Πλειάδες.[16] It probably derives fromplein (πλεῖν 'to sail') because of the cluster's importance in delimiting the sailing season in theMediterranean Sea: "the season of navigation began with theirheliacal rising".[17] In Classical Greek mythology the name was used for seven divine sisters called thePleiades. In time, the name was said to be derived from that of a mythical mother,Pleione, effectively meaning "daughters of Pleione".[18] In reality, the ancient name of the star cluster related to sailing almost certainly came first in the culture, naming of a relationship to the sister deities followed, and eventually appearing in later myths, to interpret the group name, a mother, Pleione.[19]
The M45 group played an important role in ancient times for the establishment of many calendars thanks to the combination of two remarkable elements. The first, which is still valid, is its unique and easily identifiable appearance on the celestial vault near theecliptic. The second, essential for the ancients, is that in the middle of the third millennium BC, this asterism (a prominent pattern or group of stars that is smaller than a constellation) marked thevernal point.[20] (2330 BC with ecliptic latitude about +3.5° according toStellarium)
On theNebra sky disc, dated circa 1600 BC, the cluster of seven dots in the upper right portion of the disk is believed to be the Pleiades.
The importance of this asterism is also evident in northern Europe. The Pleiades cluster is displayed on theNebra sky disc that was found in Germany and is dated to around 1600 BC. On the disk the cluster is represented in a high position between the Sun and the Moon.
This asterism also marks the beginning of several ancient calendars:
In ancient India, it constitutes, in theAtharvaveda, compiled around 1200-1000 BC, the firstnakṣatra (Sanskrit name for lunar stations), which is calledKṛttikā (क्रृत्तिका), a revealing name since it literally means 'the Cuttings',[21] i.e."Those that mark the break of the year".[22] This is so before the classic list lowers thisnakṣatra to third place, henceforth giving the first to the star coupleβ Arietis andγ Arietis, which, notably inHipparchus, at that time, marks the equinox.
In Mesopotamia, theMUL.APIN compendium, the first known Mesopotamian astronomy treatise, discovered at Nineveh in the library of Assurbanipal and dating from no later than 627 BC, presents a list of deities [holders of stars] who stand on "the path of the Moon", a list which begins with mul.MUL.[23]
In Greece, thePleiádes (Πλειάδες) are a group whose name is probably functional before having a mythological meaning, as André Lebœuffle points out, who has his preference for the explanation by theIndo-European root*pe/ol-/pl- that expresses the idea of 'multiplicity, crowd, assembly'.[24]
Similarly, the Ancient Arabs begin their oldparapegma type calendar, that of theanwā, with M45 under the name ofal-Ṯurayyā (الثريّا).[25] And this before their classic calendar, that of themanāzil al-qamar or 'lunar stations', also begins with the star coupleβ Arietis andγ Arietis whose name,al- Šaraṭān (الشرطان), is literally "the Two Marks [of entering the equinox]"[26]
Although M45 is no longer at the vernal point, the asterism still remains important, both functionally and symbolically. In addition to the changes in the calendars based on the lunar stations among the Indians and the Arabs, consider the case of an ancient Yemeni calendar in which the months are designated according to an astronomical criterion that caused it to be namedCalendar of the Pleiades: the month ofḫams, literally 'five', is that during which the Sun andal-Ṯurayyā, i.e. the Pleiades, deviate from each other by fivemovements of the Moon, i.e. five times the path that the Moon travels on average in one day and one night, to use the terminology ofAbd al-Rahman al-Sufi.[27]
Commemorative silver one dollar coin issued in 2020 by theRoyal Australian Mint - on the reverse, the Seven Sisters (Pleiades) are represented as they are portrayed in an ancient story of Australian Indigenous tradition.[28]
Galileo's drawings of the Pleiades star cluster fromSidereus Nuncius
The earliest known depiction of the Pleiades is likely a Northern GermanBronze Age artifact known as theNebra sky disk, dated to approximately 1600 BC.[39] TheBabylonian star catalogues name the PleiadesMULMUL (𒀯𒀯), meaning 'stars' (literally 'star star'), and they head the list of stars along the ecliptic, reflecting the fact that they were close to the point of thevernal equinox around the twenty-third century BC. The Ancient Egyptians may have used the names "Followers" and "Ennead" in the prognosis texts of the Calendar of Lucky and Unlucky Days of papyrus Cairo 86637.[40] SomeGreek astronomers considered them to be a distinctconstellation, and they are mentioned byHesiod'sWorks and Days,[41]Homer'sIliad andOdyssey,[42] and theGeoponica.[43] The Pleiades was the most well-known "star" among pre-Islamic Arabs and so often referred to simply as "the Star" (an-Najm;النجم).[44] Some scholars ofIslam suggested that the Pleiades are the "star" mentioned inSurah An-Najm ('The Star') in theQuran.[45]
On numerous cylinder seals from the beginning of the first millennium BC, M45 is represented by seven points, while theSeven Gods appear, on low-reliefs of Neo-Assyrian royal palaces, wearing long open robes and large cylindrical headdresses surmounted by short feathers and adorned with three frontal rows of horns and a crown of feathers, while carrying both an ax and a knife, as well as a bow and a quiver.[46]
As noted by scholarStith Thompson, the constellation was "nearly always imagined" as a group of seven sisters, and their myths explain why there are only six.[47] Some scientists suggest that these may come from observations back whenPleione was farther fromAtlas and more visible as a separate star as far back as 100,000 BC.[48]
InJapan, the cluster is mentioned under the nameMutsuraboshi ("six stars") in the eighth-centuryKojiki.[49] The cluster is now known in Japan as Subaru.[50]
It also was chosen as the brand name ofSubaru automobiles to reflect the origins of the firm as the joining of five companies, and is depicted in the firm's six-star logo.[52]
InJ. R. R. Tolkien's legendarium, whereThe Lord of the Rings is set, Pleiades is referred to as Remmirath, the netted star, as are several other celestial bodies, such as the constellation Orion as Menelvagor, swordsman of the Sky.
Galileo Galilei was the firstastronomer to view the Pleiades through atelescope.[53] He thereby discovered that the cluster contains many stars too dim to be seen with the naked eye. He published his observations, including a sketch of the Pleiades showing 36 stars, in his treatiseSidereus Nuncius in March 1610.
The Pleiades have long been known to be a physically related group of stars rather than any chance alignment.John Michell calculated in 1767 that the probability of a chance alignment of so many bright stars was only 1 in 500,000, and so surmised that the Pleiades and many other clusters must consist of physically related stars.[54] When studies were first made of theproper motions of the stars, it was found that they are all moving in the same direction across the sky, at the same rate, further demonstrating that they were related.
Charles Messier measured the position of the cluster and included it as "M45" in hiscatalogue ofcomet-like objects, published in 1771. Along with theOrion Nebula and thePraesepe cluster, Messier's inclusion of the Pleiades has been noted as curious, as most of Messier's objects were much fainter and more easily confused with comets—something that seems scarcely possible for the Pleiades. One possibility is that Messier simply wanted to have a larger catalogue than his scientific rivalLacaille, whose 1755 catalogue contained 42 objects, and so he added some bright, well-known objects to boost the number on his list.[55]
Edme-Sébastien Jeaurat then drew in 1782 a map of 64 stars of the Pleiades from his observations in 1779, which he published in 1786.[56][57][58]
The distance to the Pleiades can be used as a key first step to calibrate thecosmic distance ladder. As the cluster is relatively close to the Earth, the distance should be relatively easy to measure and has been estimated by many methods. Accurate knowledge of the distance allows astronomers to plot aHertzsprung–Russell diagram for the cluster, which, when compared with those plotted for clusters whose distance is not known, allows their distances to be estimated. Other methods may then extend the distance scale from open clusters to galaxies and clusters of galaxies, and a cosmic distance ladder may be constructed. Ultimately astronomers' understanding of the age and future evolution of the universe is influenced by their knowledge of the distance to the Pleiades. Yet some authors argue that the controversy over the distance to the Pleiades discussed below is ared herring, since the cosmic distance ladder can (presently) rely on a suite of other nearby clusters where consensus exists regarding the distances as established by theHipparcos satellite and independent means (e.g., theHyades, theComa Berenices cluster, etc.).[3]
Animation of proper motion in 400,000 years—cross-eyed viewing (click for viewing guide)
Measurements of the distance have elicited much controversy. Results prior to the launch of theHipparcos satellite generally found that the Pleiades were approximately 135parsecs (pc) away from Earth. Data fromHipparcos yielded a surprising result, namely a distance of only 118 pc, by measuring theparallax of stars in the cluster—a technique that should yield the most direct and accurate results. Later work consistently argued that theHipparcos distance measurement for the Pleiades was erroneous:[3][4][5][59][60][61] In particular, distances derived to the cluster via theHubble Space Telescope and infraredcolor–magnitude diagram fitting (so-called "spectroscopic parallax") favor a distance between 135 and 140 pc;[3][59] a dynamical distance fromoptical interferometric observations of the inner pair of stars withinAtlas (a bright triple star in the Pleiades) favors a distance of 133 to 137 pc.[61] However, the author of the 2007–2009 catalog of revisedHipparcos parallaxes reasserted that the distance to the Pleiades is ~120 pc and challenged the dissenting evidence.[2] In 2012, Francis and Anderson[62] proposed that a systematic effect onHipparcos parallax errors for stars in clusters would bias calculation using theweighted mean; they gave aHipparcos parallax distance of 126 pc andphotometric distance of 132 pc based on stars in theAB Doradus,Tucana-Horologium andBeta Pictoris moving groups, which are all similar in age and composition to the Pleiades. Those authors note that the difference between these results may be attributed to random error.More recent results usingvery-long-baseline interferometry (VLBI) (August 2014), and preliminary solutions usingGaia Data Release 1 (September 2016) and Gaia Data Release 2 (August 2018), determine distances of 136.2 ± 1.2 pc,[63] 134 ± 6 pc[64] and 136.2 ± 5.0 pc,[65] respectively. The Gaia Data Release 1 team were cautious about their result, and the VLBI authors assert "that theHipparcos-measured distance to the Pleiades cluster is in error".
The most recent distance estimate of the distance to the Pleiades based on theGaia Data Release 3 is135.74±0.10 pc.[66]
The cluster core radius is approximately 8light-years andtidal radius is approximately 43 light-years. The cluster contains more than 1,000 statistically confirmed members, not counting the number that would be added if allbinary stars could be resolved.[68] Its light is dominated by young, hotblue stars, up to 14 of which may be seen with the naked eye, depending on local observing conditions and visual acuity of the observer. The brightest stars form a shape somewhat similar to that ofUrsa Major andUrsa Minor. The total mass contained in the cluster is estimated to be approximately 800solar masses and is dominated by fainter and redder stars[clarification needed].[68] An estimate of the frequency ofbinary stars in the Pleiades is approximately 57%.[69]
The cluster contains manybrown dwarfs, such asTeide 1. These are objects with less than approximately 8% of theSun's mass, insufficient fornuclear fusion reactions to start in their cores and become proper stars. They may constitute up to 25% of the total population of the cluster, although they contribute less than 2% of the total mass.[70] Astronomers have made great efforts to find and analyze brown dwarfs in the Pleiades and other young clusters, because they are still relatively bright and observable, while brown dwarfs in older clusters have faded and are much more difficult to study.
Stars of Pleiades with color and 10,000-year backward proper motion shown
Ages for star clusters may be estimated by comparing theHertzsprung–Russell diagram for the cluster with theoretical models ofstellar evolution. Using this technique, ages for the Pleiades of between 75 and 150 million years have been estimated. The wide spread in estimated ages is a result of uncertainties in stellar evolution models, which include factors such asconvective overshoot, in which aconvective zone within a star penetrates an otherwise non-convective zone, resulting in higher apparent ages.[citation needed]
Another way of estimating the age of the cluster is by looking at the lowest-mass objects. In normalmain-sequence stars,lithium is rapidly destroyed innuclear fusion reactions.Brown dwarfs can retain their lithium, however. Due to lithium's very low ignition temperature of 2.5 × 106 K, the highest-mass brown dwarfs will burn it eventually, and so determining the highest mass of brown dwarfs still containing lithium in the cluster may give an idea of its age. Applying this technique to the Pleiades gives an age of about 115 million years.[72][73]
The cluster isslowly moving in the direction of the feet of what is currently the constellation ofOrion. Like most open clusters, the Pleiades will not stay gravitationally bound forever. Some component stars will be ejected after close encounters with other stars; others will be stripped by tidal gravitational fields. Calculations suggest that the cluster will take approximately 250 million years to disperse, because of gravitational interactions withgiant molecular clouds and thespiral arms of our galaxy hastening its demise.[74]
With larger amateur telescopes, the nebulosity around some of the stars may be easily seen, especially when long-exposure photographs are taken. Under ideal observing conditions, some hint of nebulosity around the cluster may be seen even with small telescopes or average binoculars. It is areflection nebula, caused by dust reflecting the blue light of the hot, young stars.
It was formerly thought that the dust was left over from theformation of the cluster, but at the age of approximately 100 million years generally accepted for the cluster, almost all the dust originally present would have been dispersed byradiation pressure. Instead, it seems that the cluster is simply passing through a particularly dusty region of theinterstellar medium.[11]
Studies show that the dust responsible for the nebulosity is not uniformly distributed, but is concentrated mainly in two layers along the line of sight to the cluster. These layers may have been formed by deceleration due toradiation pressure as the dust has moved toward the stars.[75]
Analyzing deep-infrared images obtained by theSpitzer Space Telescope andGemini North telescope, astronomers discovered that one of the stars in the cluster,HD 23514, which has a mass and luminosity a bit greater than that of the Sun, is surrounded by an extraordinary number of hot dust particles. This could be evidence for planet formation around HD 23514.[76]
^abcdMajaess, Daniel J.; Turner, David G.; Lane, David J.; Krajci, Tom (2011). "Deep Infrared ZAMS Fits to Benchmark Open Clusters Hosting delta Scuti Stars".Journal of the American Association of Variable Star Observers (Jaavso).39 (2): 219.arXiv:1102.1705.Bibcode:2011JAVSO..39..219M.
^Wilfred G. Lambert, (en) « The section AN », in : Luigi Cagni (a cura di),Il bilinguismo a Ebla, Atti del convegno inter-nazionale (Napoli,, 19-22 aprile 1982), Napoli, Istituto Universitario Orientale, Dipartimento di studi asiatici, XXII (1984), 396-397
^Pingree, David; Morrissey, Patrick (June 1989). "On the Identification of the Yogataras of the Indian Naksatras".Journal for the History of Astronomy.20 (2): 100.doi:10.1177/002182868902000202.
^(en) André Le Bœuffle,Les Noms latins d'astres et de constellations, éd. Paris: Les Belles Lettres, 1977, pp. 120-124.
^Charles Pellat,Dictons rimés,anwa et mansions lunaires chez les Arabes, inArabica. Journal of arabic and islamic studies, vol. 2 (1955) p. 19.
^Roland Laffitte,Essai de reconstitution du comput antique, et« le comput des desmanāzil al-qamar ou stations lunaires, inLe ciel des Arabes. Apport de l'uranographie arabe, Paris : Geuthner, 2012, pp. 42-43, puis 51-60.
^(de) Eduard Glaser,Die Sternkunde der südarabischen Kabylen, Wien : aus der Hof- und Staatsdruckerei, (s.d.) [Aus dem XCL. Bande der Sitzb. der kays. Akad. der Wissensch., II. Jänner-Heft Jahrg.1885], pp. 3-4.
^James Hastings; John Alexander Selbie; Andrew Bruce Davidson; Samuel Rolles Driver; Henry Barclay Swete (1911).Dictionary of the Bible: Kir-Pleiades. Scribner. pp. 895–896.
^Saqib Hussain, "The Prophet's Vision in Sūrat al-Najm,"Journal of the International Qur'anic Studies Association,5 (2020): 97–132.
^Jeremy Black & Anthony Green,Gods, Demons and Symbols of Ancient Mesopotamia, an Illustrated Dictionary, London: British Museum Press, 1992, p. 162.
^Thompson, Stith (1977).The Folktale. University of California Press. pp. 237-238.ISBN0-520-03537-2.
^Norris, Ray P., Norris, Barnaby R.M. (2021). Why Are There Seven Sisters?. In: Boutsikas, E., McCluskey, S.C., Steele, J. (eds) Advancing Cultural Astronomy. Historical & Cultural Astronomy. Springer, Cham.https://doi.org/10.1007/978-3-030-64606-6_11
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